In the exploration for oil and gas, it is often desirable to determine the age of clay minerals in sedimentary rocks. This information can be used as an aid to determine the genesis, history, and migration pattern of hydrocarbons, fluids, and minerals in sediments. Since sedimentary rocks often contain illite, considerable effort has been made by geologists to determine the date illite was formed in a rock sample.
Various dating techniques have been suggested. A widely used dating technique is known as the potassium(K)-argon(Ar) dating method. This dating technique is based on the principle that when a K-bearing mineral, such as illite, was formed it contained .sup.40 K, one of three isotopes of potassium. The .sup.40 K then underwent radioactive decay according to a constant decay rate to produce two daughter products, .sup.40 Ca and .sup.40 Ar. Because Ar is an inert element, it is very unlikely to be taken up into newly formed minerals. Therefore, most if not all argon in a mineral is a result of radioactive decay. The most abundant calcium isotope is .sup.40 Ca, and calcium is more abundant than potassium in the earth's crust. For these reasons, age dating is more reliable using the daughter argon isotope than the daughter calcium isotope even though 89% of all .sup.40 K decay produces .sup.40 Ca. The amount of .sup.40 Ar and .sup.40 K in a sample is determined directly with a mass spectrometer and by inference based on measurement of the total potassium content.
The K-Ar dating technique is relatively simple to use if at the time the earth sample was formed it was not contaminated with previously formed argon. Because argon is an inert gas, it escapes easily from molten rock and begins to accumulate only after the rock solidifies and minerals cool. For this reason the initial argon present in molten rock as it forms is generally not a problem in K-Ar dating. When molten magma was deposited on the earth's surface, any argon that was present in the magma would have been lost to the earth's atmosphere. The magma would have been so hot that the argon gas would have escaped. As the magma cooled it crystallized and trapped potassium ions in the crystalline lattice. The .sup.40 K began to decay to .sup.40 Ar and the gaseous .sup.40 Ar was trapped in the crystal lattice. The age of the sample can be easily determined by comparing the amount of potassium to the amount of argon.
The K-Ar dating technique is more difficult to use if the rock has undergone any chemical alteration or post-formational heating (above about 250.degree. C.). Such heating or chemical alteration may cause some or all of the accumulated argon to escape and thus partially or totally reset the K-Ar clock. Because sedimentary rocks have typically not undergone heating above 250.degree. C., application of K-Ar dating to sedimentary rocks does not suffer from this problem. However, when the K-Ar dating technique was first introduced in the 1950s, it was generally believed the technique was not practical for dating illite found in sedimentary rocks because such rocks contain a mixture of "old" and "new" illite.
As sediments undergo heating during burial, new illite is often formed in the rocks, especially in shales. This newly formed illite is called "diagenetic illite". Since illite is rich in potassium, K-Ar dating can be used to determine when the diagenetic illite was formed. However, the resulting age measured using K-Ar dating can be inaccurate because rocks in which the diagenetic illite forms often also contain old illite which is called "detrital illite". The detrital illite may have been carried there by wind, ocean currents, streams, or rivers millions or hundreds of millions of years before the diagenetic illite was formed.
Because the diagenetic illite formed in response largely to temperature, the amount of it and its age are important indicators of the sediment's thermal history. This information is useful in hydrocarbon exploration because it helps establish the time of maturation of the source rock for oil and gas. Knowing the age of the detrital illite can also help in determining the provenance of the sedimentary rock.
A major difficulty in using the K-Ar dating method on an illite sample that contains both diagenetic illite and detrital illite is determining the relative contribution of each illite component. Since diagenetic and detrital illites were formed at different times, K-Ar dating of the sample would give a mean age of all illite in the sample. Since detrital illite can contain much more argon than the more abundant diagenetic illite, the detrital illite may dominate the age value disproportionately.
Determining sedimentary (diagenetic) age using K-Ar dating is further complicated by the fact that different sized particles in the rock seldom have identical proportions of detrital illite and diagenetic illite. Coarser-sized fractions tend to have K-Ar age values greater than the age of the finer-sized fractions.
Efforts have been made to physically separate diagenetic illite from the detrital illite, but such efforts have not been entirely successful. Even the finer-sized fractions appear to contain mixtures of the two illite components. Therefore, the dates obtained using K-Ar dating are often not useful.
A need exists for an improved process for using the K-Ar dating method to determine the age of detrital illite and the age of diagenetic illite in an earth sample that contains both diagenetic illite and detrital illite.